1. Field of the Invention
The present invention relates to portable chain saw equipment and in particular relates to an improved chain saw brake control and actuating mechanism for maintaining the brake in disengaged condition during normal operation and for automatically engaging the brake to protect the operator from the cutting chain in the event of contact between the operator and the safety lever which may occur as a result of "kickback."
2. Description of the Prior Art
A manually operated, portable chain saw commonly includes an engine carrying housing, a guide bar projecting forwardly from the housing and an engine driven cutter chain mounted on the guide bar for continuous movement. Proper operation of the chain saw requires that an operator grasp the machine with both hands, one hand holding a gripping handle mounted on the housing, and the other hand engaging a housing control handle for actuating a trigger throttle mechanism.
A well known safety problem in the use of a manually operated portable chain saw of this type is the occurrence of "kickback." Kickback is characterized by a violent movement of the saw which may occur when the nose of the guide bar and cutting chain contacts a solid object. The cutting chain of the saw tends to bite into the solid object with a force proportional to the speed of the chain. At the same time, there is a corresponding reaction force exerted on the saw by the object. If the momentum of the chain and other moving parts of the saw carries the chain into the object, then the energy of the chain is expended in the cutting action and the reaction force on the saw is greatly reduced. However, at lower chain speeds, the momentum may not be sufficient to sustain the cutting action, and the chain may become siezed within the object. If the chain is not carried on through the cut, the saw is suddenly subjected to a full reactive force. When this force is exerted at the middle or on the upper quadrant of the nose of the guide bar and cutting chain, the nose moves suddenly upward in rotation generally about the rear of the saw.
Because of the way that a chain saw is held, the kickback motion is extremely difficult for an operator to control; thus there is considerable risk that he will be hit by the moving chain. If the saw is being held horizontally at the time of the kickback, then its motion will be somewhat opposed by the weight of the saw. This opposition decreases substantially as the saw is held more nearly vertically. For this reason, perhaps the most dangerous kickback situation is when the saw is being used to cut overhanging limbs or other such objects. There may be additional lack of control in this situation if the operator's arms are extended to reach the object above.
A number of safety brakes have been developed for quickly stopping the chain when kickback occurs. Although these safety brakes and their actuating mechanisms represent significant advancements, room for further improvement in the art remains. For example, there is a continuing interest in reducing the reaction time of the actuating and braking mechanism, and in reducing the number and size of the mechanical components required for carrying out the actuating and braking mechanism in order to minimize the weight and size of the manually portable chain saw.
Therefore, it is an important object of the invention to provide a chain saw actuating and braking apparatus which is capable of imposing sufficient braking forces to stop the cutter chain in less time than it takes a violent kickback reaction to occur.
It is another object of the invention to provide a chain saw braking mechanism which operates reliably, effectively and only in response to positive actuation of a safety lever.
It is yet another object of the invention to provide a chain saw brake actuating mechanism which involves a minimal number of parts that are capable of compact arrangement.